The present invention relates to a workpiece-bore processing apparatus and method wherein a processing ball with the same outer diameter as an inner diameter of a bore of a workpiece or a diameter slightly larger than the diameter of the bore is inserted through the bore under pressure to finish an inner surface of the bore.
Recently, with the rising trend in the speed and precision requirements with respect to bearings used in office equipment and household appliances, there has been an increasing need for fluid bearings of the dynamic pressure type and also for special types of sleeve bearings processed to special precision requirements. The present invention can be applied to, for example, sleeve bore finishing with respect to high precision bearings and, more particularly, to a bearing bore processing apparatus and method for finishing a sleeve bore of a bearing to high precision by controlled special plastic working and without using cutting and abrading techniques.
The construction of a conventional bearing bore processing apparatus is illustrated in FIGS. 8 to 10 of the accompanying drawings. In the figures, element 11 is a sleeve of a bearing as one example of a workpiece; element 11A is a bearing bore; element 12 is a dynamic pressure generating groove; element 13 is a residual projection; element 14 is a processing ball formed of a material such as hardened steel, and element 15 is a pin.
Conventionally, the process of bore processing with respect to precision bearings, such as dynamic pressure type grooved fluid bearings, is carried out in such a way that as FIG. 8 illustrates, the bearing bore 11A of the sleeve 11 is processed through a cutting process and otherwise to a diametrical size D1, and thereafter by forming a plurality of dynamic pressure generating grooves 12 by an unillustrated processing tool (e.g., a processing apparatus as shown in Japanese Patent Publication No. 3-68768). Residual projections 13 which may be produced during this processing are removed by a so-called ball burnishing technique such that a processing ball 14 having a given diameter Db which is made of hardened steel or the like is thrustingly forced through the bore by the pin 15 as illustrated in FIG. 9.
As a result of such a ball burnishing process, the bore of the sleeve 11 is finished to a diameter D2 slightly larger than the inner diameter D1 of a roughly-processed bore. In FIG. 10, the bore 11A of the sleeve 11 is structurally less tough at positions adjacent opposite ends thereof, whereat it involves considerable plastic deformation due to the ball burnishing process as shown by D3. This naturally results in unsatisfactory cylindricity of the bore.
Further, with the foregoing arrangement, as shown in FIG. 11, the roughly-processed bore diameter D1 of the sleeve 11 involves a variation on the order of 6 .mu.m relative to the reference diameter of the bore (e.g., in the range of from -5 to +1 .mu.m relative to the reference diameter) depending upon the cutting process conditions involved, and accordingly, the finished diameter D2 may involve a variation of the same order as that of the roughly-processed bore diameter. From the standpoint of mass production, therefore, it has hitherto been extremely difficult to finish the bore diameter to tolerance of about 1 to 3 .mu.m (specifically, in the range of from -1 to +1 .mu.m) which is required for provision of a precision bearing sleeve.